1. Technical Field
The present invention relates to a rear projector and a projection system and, more specifically, to a scintillation suppression technology for a rear projector and a projection system.
2. Related Art
In recent years, projectors have rapidly become widely available. The projectors are of two types, i.e., front projector and rear projector. The front projectors are those mainly used for presentation use, and recently the rear projectors are being acknowledged as one form of a large-screen display. The biggest advantage of a projection-type image display device is the lower price in size wise compared with a direct-view-type display such as liquid crystal television system and plasma display. However, the direct-view-type display is also recently reduced in price, and thus the image display device of projection type is required to have much higher image quality characteristics. The projector displays images by irradiating an optical modulation element such as liquid crystal light valve with lights emitted from a light source, and then by extending, for projection onto a screen, the lights being the results of modulation by the optical modulation element. At the time of such image display, a viewer not only sees images displayed on the screen but also finds the screen glaring in its entirety. This glaring is caused by varying intensity as a result of beam interference, and is called speckle noise or scintillation.
As to scintillation, the generation principle thereof is now briefly described.
As shown in FIGS. 13A and 13B, a light coming from a light source 70 passes through a liquid crystal light valve (not shown), and then is projected on a screen 74. The light projected on the screen 74 is diffracted by a plurality of scattering materials 72 included in the screen 74, and then is diffused by these scattering materials 72 behaving like a secondary wave source. As shown in FIG. 13B, two spherical waves from the secondary wave source are increased or reduced in intensity depending on the phase relationship therebetween so that light and dark fringes (interference fringes) appear between the screen 74 and a viewer. When the viewer's eyes are focused on the screen surface S where the interference fringes are observed, the viewer acknowledges the interference fringes as scintillation of glaring the projected images. The scintillation annoys the viewer who wants to look at the images on the screen as if there is a veil, a lace cloth, a spiderweb, or others between him or her and the screen surface. With scintillation occurred as such, the viewer sees double images of images on the screen and the scintillation. The viewer thus tries to focus on both of these, and resultantly gets very fatigued. As such, scintillation causes suffering to the viewer.
For use with the recent projector, a new type of light source has been under development as an alternative to the previous high-pressure mercury lamp. Especially a laser light source is highly expected to serve as a light source for use with a next-generation projector in terms of energy efficiency, color reproducibility, long life, instantaneous illumination, and others. The issue here is that lights coming from a laser light source are highly coherent on the screen with alignment of beam phase in any adjacent areas. The laser lights sometimes have the coherent length of several tens of meters, and if any one light source is divided and then combined together again, the resulting lights synthesized through an optical path being shorter than the coherent length cause strong interference. As a result, the resulting scintillation (interference fringes) is clearer than that by a high-pressure mercury lamp. In consideration thereof, reducing the scintillation is an essential technology especially in the course of making practical a projector using a laser light source.
As measures to be taken to reduce such scintillation, the following technologies have been proposed.
JP-A-11-038512 describes the technology of optimizing the diffusibility of a screen, which is of three-layer configuration including a diffusion layer, a transparent layer, i.e., lenticular lens, and another diffusion layer. With the diffusion layer complicated in configuration as such, the randomness is increased for irregular portions resulted from interference. Accordingly, with an increase of any small components in the irregular portions as a result of interference, i.e., interference fringes with low space frequency, when any movement of line of sight is observed, the interference fringes are integrated and averaged by the persistence of human vision so that there are effects that the interference fringes disappear. Especially for viewing of moving images, the line of sight moves frequently so that the reduction effects can be expected for scintillation.
JP-A-2001-100316 describes a screen of making temporal changes, i.e., shape, relative positional relationship, and index of refraction, to a light scattering element in a light scattering layer. Such changes are made by applying light, electric field, magnetic field, heat, stress, and others, to the light scattering layer. With temporal changes made as such to the scattering distribution and phase of scattering waves of the light diffusion layer, the effects can be expected for occurrence prevention of scintillation.
The problem with the technology of JP-A-11-038512 is that the scattering surface located at the end is fixed in scattering state, and this means that the distribution of beam phase is also fixed in a space between the screen and a viewer. The beam phase is the one caused by the interference between beams emitted from each point on the scattering surface. The irregular portions resulted from interference are thus acknowledged as a fixed image, and the irregular portions resulted from interference do not completely disappear unless the line of sight moves frequently. If this is the case, there are almost no effects with a projector equipped with a high-coherent laser light source. That is, no matter how much the scattering level is increased for the screen, if a viewer sees the screen with his or her eye point fixed, the viewer notices the interference fringes. Moreover, with such measures attempted to take by increasing the scattering level, there is also a possibility of causing image blurring, and thus such measures are not serving good enough to achieve the fundamental object of high image quality.
The method described in JP-A-2001-100316 is of making light scattering elements undergo the Brownian movement by varying the temperature of a light scattering layer, which is sandwiched by electrode plates each being a transparent resistance film or others. JP-A-2001-100316 also describes several other methods of making changes to the light scattering element in terms of shape and index of refraction by application of electric field, magnetic field, or others. However, such methods require a great deal of energy and time until such changes, i.e., shape, relative positional relationship, and index of refraction, are made to the light scattering element, and this is not considered efficient at all. Also with such drive means described above, the energy transmission efficiency is low with respect to the light scattering layer, and generation of vibration, noise, unwanted electromagnetic waves, exhaust heat, and others, possibly inhibits the viewing environment from being comfortable.